1. Field of the Invention
The present invention relates to zinc ferrite colour pigments, to a process for preparing them and to their use. In particular, the present invention relates to light-coloured zinc ferrite colour pigments having enhanced thermal stability. As used herein and in the claims the word “color” and the British variant thereof, “colour”, shall be used interchangeably and understood to have the same meaning.
2. Brief Description of the Prior Art
Depending on its stoichiometric composition, additives, particle sizes, crystal form and surface properties zinc ferrite, which crystallizes in the spinel lattice, can be used as a starting material for soft magnets, as a corrosion preventative or as a colour pigment. In this regard, the term “tan” pigments has been used for non-ferrimagnetic colour pigments.
U.S. Pat. No. 3,832,455 describes the preparation of zinc ferrite pigments wherein a precipitate of iron oxide hydroxide from iron(II) sulphate solution on zinc oxide or zinc carbonate at a pH of from 5 to 6 and a temperature of from 49 to 52° C. is filtered and the solids are washed, dried and calcined.
U.S. Pat. No. 2,904,395 describes the preparation of zinc ferrite pigments either by co-precipitation from the corresponding iron-containing and zinc-containing solutions with subsequent filtration, washing, drying and calcining or else by calcining of an intimate mixture of iron oxide hydroxide and zinc oxide obtained in aqueous suspension. Calcining takes place at temperatures up to 1000° C. with the addition of catalysts, hydrochloric acid or zinc chloride for example.
U.S. Pat. No. 4,222,790 describes how the calcining operation for preparing zinc ferrite or magnesium ferrite can be improved by adding alkali metal silicate to the mixture. As a flocculent for the filtration it is possible to add aluminium sulphate.
The addition of Al2O3— and P2O5— forming compounds during the calcination of chloride-free zinc ferrite pigments of high colour purity is described in DE-A 31 36 279.
Zinc ferrite pigments of high colour purity without additives can be obtained in accordance with EP-B 1 54 919 by using acicular α-FeOOH of defined particle size and surface area and zinc oxide of defined surface area.
To improve the cooling characteristics after calcining, boric acid or boron phosphates are employed.
In JP-B 5 70 11 829, titanium oxide is added for preparing heat-resistant yellow zinc ferrite pigments.
Finally, T.C. Patton in Pigment Handbook, Vol. 1, Properties and Economics, pp. 347 and 348, John Wiley & Sons, New York 1973 describes the anisometric zinc ferrites.
Since they are distinguished by outstanding light stability and weather stability and also by high thermal stability zinc ferrites are used instead of less heat-stable mixtures of yellow iron oxide and red iron oxide.
Also, they have found application in particular for the colouring of sand granules, lime sandstones, enamels, ceramic glazes, baking enamels and plastics.
Despite the fact that the zinc ferrite pigments are prepared at high temperatures, above 700° C., in a variety of systems they are subject to varying levels of colour change at relatively lower temperatures, particularly when used for colouring organic materials. In these systems, therefore, they can frequently no longer be regarded as thermally stable. In the colouring of plastics, for instance, it is found that above about 250° C. the shift in shade towards darker, dirty tones is already so marked that colouring with zinc ferrite pigments is no longer rational. This is particularly the case with the colouring of plastics which require relatively higher processing temperatures, such as polyamide or ABS plastics, for example.
DE-A 3 819 626 describes the preparation of thermally stable zinc ferrite colour pigments, a process for preparing them and their use. By adding a lithium compound to an initial mixture of zinc oxide and iron oxide, zinc ferrite colour pigments are prepared which contain lithium. When tested for heat resistance in PE-HD in accordance with DIN 53 772 these zinc ferrite colour pigments have a colour difference according to DIN 6174 of 2.8 ΔE* units from reference samples produced at 260° C. and at the lowest possible test temperature (=200° C.). At processing temperatures of 300° C. the result was 5.0 ΔE* units. In the colour locus, however, these pigments are darker in lightness by more than 2.5 L* units than undoped zinc ferrites.
It was an object of the invention to provide further zinc ferrite colour pigments which have a good and preferably better thermal stability than those of the prior art and are preferably light in colour.